At the outset a distinction is to be made between functional ankle braces and therapeutic ankle orthoses. An ankle brace is designed to limit the range of side-to-side foot motion during stressful functional activities. An ankle orthosis, on the other hand, serves as a functional ankle brace during the transitional period between resolution of acute sprain symptoms and complete restoration of normal ankle function and at the same time therapeutically treats and controls residual sprain symptoms.
An example of an ankle brace of the prior art is that disclosed in U.S. Pat. No. 5,069,202. Its structure includes a foot shell pivoted to a leg shell which comprises lateral and medial uprights connected by an anterior band. Adjustable strap means are included for holding these components in operative position on the leg, ankle and foot. Another form of ankle brace is disclosed in U.S. Pat. No. 5,199,941 which includes medial and lateral shells connected to a foot shell, though without a pivotal joint between them. A therapeutic ankle orthosis is disclosed in U.S. Pat. No. 4,556,054 and includes a foot shell and a leg cuff interconnected by appropriate adjustable straps.
That prior art disclosure perhaps most pertinent to the present invention is German Gebrauschmuster (or Petty Patent) No. 6 88 14 157.8 published on Feb. 9, 1989. It is intended to function therapeutically during recovery from acute sprain symptoms and at the same time to brace the ankle to limit the maximum range of side-to-side foot motion. The therapeutic functions include compression and joint stabilization. This German Petty Patent teaches a foot shell pivotally and slidably connected to the lower end of lateral and medial shells by means of a post in a curved slot, with the components held together by adjustable strap means. The lateral shell includes an aperture for encircling the fibular malleolus and an inner pad for compression. The pad is not of U-shape, though pads of that configuration are known from such prior art patents as my U.S. Pat. No. 4,590,932 to disperse edema upwardly from those regions around the fibular malleolus where excess fluid accumulates.
The most common mechanism causing ankle ligament injury is inversion or turning inward of the sole of the foot. However, there is an associated mechanism which involves rotation of the foot and leg in opposite directions in a horizontal plane, referred to as adduction of the foot or internal foot rotation combined with external leg rotation. Prior art ankle braces emphasize means for restraining inversion of the foot but do little to protect against external rotation of the leg upon a foot that is fixed to the ground.
In the German Petty Patent referred to above the articulation of the foot shell with respect to the lateral shell by means of a curved slot and rivet allows for a combination of rotational and gliding movement between the components. The curvature of the slot does not correspond to the functional axis of the upper ankle joint, namely the talocrual joint. Rotation of the foot and leg in opposite directions in a horizontal plane would result in a spreading apart of the lateral and foot components of the German Petty Patent along the anterior margin of the overlapped areas. Such a design does not provide resistance to rotary ankle motion within the horizontal plane.